1. Field of the Invention
The present invention relates to compressible elements for use with rotatable elements, such as dicing saw elements, installable onto a shaft or spindle or for use with shafts or spindles upon which rotatable elements are to be installed. More specifically, the compressible elements of the present invention are useful for centering rotatable members around the rotational axis of shafts or spindles, for reducing clearances between the inner diameters or surfaces of rotatable members and the outer diameters or surfaces of shafts or spindles, as well as for dampening vibrations during rotation of such a shaft or spindle and, thus, each of the rotatable elements thereon. In addition, the present invention includes methods for installing one or more rotatable elements upon a shaft or spindle, as well as to methods for designing rotatable elements to be installed upon shafts or spindles and shafts or spindles upon which rotatable elements are to be installed.
2. State of the Art
Many devices utilize high-speed spinning elements. For instance, dicing saws for cutting circuit boards or other carrier substrates, semiconductor substrates (e.g., silicon wafers), and the like employ rotating members that may be spun at rotational speeds of up to 60,000 revolutions per minute (“rpm”).
In order to singulate or perform “dicing” operations on a carrier substrate, such as a ball grid array (“BGA”) assembly substrate, or another semiconductor device substrate, such as a full or partial wafer of silicon or another substrate from which a semiconductor device may be singulated, dicing saws are typically used. Apparatus for dicing BGA substrates usually comprise at least one saw blade attached to a spindle, which is rotated via a motor attached thereto. Conventionally, a single saw blade was attached to a shaft or spindle and, during rotation thereof, moved repeatedly in a linear motion to cut the carrier substrate at desired locations (e.g., between adjacent BGA substrates). The use of a single saw blade to cut a substrate at a number of different, parallel locations is, however, a somewhat time-consuming process. Increasingly rapid dicing methods include use of multiple or “ganged” saw blade assemblies, which are configured to simultaneously cut a substrate at different parallel locations. In addition, due to ever-increasing densities of semiconductor devices or circuit boards on a substrate, the individual saw blades are relatively thin to accommodate narrow spacing between adjacent semiconductor devices or circuit boards. Further, in order to exact precision cuts with reduced forces on the BGA substrate during cutting, dicing blades are rotated at relatively high speeds, typically about 15,000 rpm to about 25,000 rpm and up to 60,000 rpm.
Because of such high rotational speeds, mounting and balance of a dicing saw assembly are also of concern, as mounting and balance directly affect deflection, vibration, and run-out (i.e., movement of the cutting edge out of the desired plane) during use of a dicing saw assembly for cutting. To obtain trueness and stability during cutting and to minimize run-out, the blades are typically mounted between adjacent support flanges so that only a small cutting edge of each blade is exposed. In addition, spacers may be used in a ganged assembly to provide desired spacing between the adjacent blades. Each element of the assembly must be balanced to reduce vibration during operation.
Machinery with rotatable components, such as dicing saws, are often designed with a drive shaft. Generally, a drive shaft is attached to a motor directly and provides rotational force to an assembly. Rotatable elements and other elements are generally installed onto the shaft or spindle by way of a hole formed through such elements to facilitate sliding these elements onto the shaft. Thus, the outer surface of the shaft is surrounded by the inner surface of the hole formed through the element. An engagement structure must also be provided so that torque is transmitted from the shaft or spindle to the element, causing the element to spin as the shaft or spindle is rotated. Some common engagement structures include splines, keyways, and pins. Some assemblies may alternatively be compressed onto a tapered surface or expanding fixture, as known in the art. As another alternative, engagement between an element and a shaft or spindle may be accomplished via threaded connections.
Undesired imbalance forces may develop in high-speed spinning assemblies due to radial imbalance in the mass of the spinning elements relative to the axis of rotation. Such imbalance force is determined by the eccentricity (i.e., the radial distance away from the axis of rotation) of the center of mass or gravity, as well as the magnitude of the mass of the rotating element and the square of the rotational speed of the shaft or spindle. Stated another way, imbalance force is proportional to the distance that the center of mass of the member is displaced from the rotational axis, the magnitude of the mass of the member, and the square of the rotational speed of the shaft or spindle. Therefore, relatively small center of mass eccentricities relative to the axis of rotation and masses can be greatly magnified when a shaft or spindle is rotated at high speed.
Imbalanced rotating elements may cause significant problems. Vibration of rotating elements may be destructive to rotating machinery, and imbalances may cause stresses that are typically induced upon rotating machinery to be magnified. For example, increased reciprocating stresses reduce the fatigue life of rotating machinery, causing premature failure.
Referring to FIGS. 1A and 1B, a rotatable element 20 is installed on a shaft 10. A clearance 3 exists between an outer surface 16 of the shaft 10 and an inner surface 26 of the rotatable element 20. Clearance 3 is exaggerated to illustrate the prior art. Importantly, the center of mass 21 of the rotatable element 20 does not coincide with, and may not even be near, the rotational axis 15 of the shaft 10 and, thus, of the assembly. In addition, the center of mass 21, assuming homogeneous material properties, will occur at an appropriate thickness with respect to the geometry of the longitudinal axis shown in FIG. 1B. Therefore, when the rotatable element 20 is rotated about the rotational axis 15 of the shaft 10, the center of mass 21 is offset from the rotational axis 15 by an offset distance 1. Offset distance 1 is equal to about twice the average clearance 3 between inner surface 26 of the rotatable element 20 and the outer surface 16 of the shaft 10.
The rotatable element 20 does not rotate relative to the shaft 10. Although clearance 3 may be sufficient to allow for such rotation between the rotatable element 20 and the shaft 10, the rotatable element 20 is fixed to the shaft 10 to accept torque therefrom. Typically, such fixturing is accomplished by way of a longitudinal compression element, such as a screw, bolt, or pin that extends through the inner surface 26 of the rotatable element 20 and that may be rigidly biased against the outer surface 16 of the shaft 10. Therefore, once each rotatable element 20 is secured to the shaft 10, its position along the shaft 10 and relative to other elements along the shaft 10 may be maintained.
In order to reduce the detrimental effects of imbalanced rotating machinery, spinning assemblies require frequent balancing of one or more of the rotatable elements thereon. The amount and location of imbalance of a rotating assembly may be measured, facilitating the calculation of a correction. Balance corrections may be accomplished by moving the center of mass of one or more rotatable elements to or toward the axis of rotation. Alternatively, a balance correction may be effected by adding weight at a radial distance away from the rotational axis at substantially 180° phase shift with respect to the angular position of the imbalance. As another alternative, a correction for imbalance may be accomplished by removing material at a radial distance from the rotational axis of the member at substantially the angular position of the imbalance. Obviously, position of the correction as well as magnitude of the correction weight must be taken into account to properly balance a rotating body. Further, several balancing correction operations may be required to accurately balance a rotatable element.
As imbalance variables become increasingly small and, therefore, difficult to measure and correct, assembly and reassembly of a rotatable assembly may change the balancing characteristics thereof. Thus, minor dimensional changes and clearances may cause detrimental imbalance.
Assuming that the rotatable elements are substantially balanced prior to or upon installation thereof onto a shaft, imbalance of an assembly may be caused by the clearance between the inner surface of each rotatable element and the outer surface of the shaft or spindle that is required to install the rotatable element onto the shaft or spindle. More specifically, as the rotatable elements are installed onto a shaft or spindle, the inner surface of the rotatable member does not fully engage the outer surface of the shaft or spindle. Thus, if the shaft or spindle has a circular cross section taken transverse to the axis of rotation thereof, the center of mass of the rotatable element may be offset by twice the average clearance between the inner surface of the shaft or spindle and the outer surface of the rotatable element. Alternatively, if the shaft or spindle has a noncircular cross section, the distance that the rotatable element may shift relative to the axis of rotation of the shaft or spindle may be larger than twice the average radial clearance between the inner surface of the rotatable element and the outer surface of the shaft or spindle.
The “sloppiness” that may be caused when there is too much clearance between the inner surface of a rotatable element and the outer surface of a shaft or spindle upon which the rotatable element is installed has typically been addressed by reducing the clearance between the rotatable element and the shaft or spindle. In so doing, both the shaft and rotating member must be machined more accurately to smaller tolerances, causing an increase in the cost of each. Furthermore, some clearance is required if simple assembly and disassembly are desired.
U.S. Pat. No. 5,261,385 to Kroll (hereinafter “Kroll”) teaches an exemplary abrasive cutting blade assembly with multiple, ganged blades. A hub or ring is positioned onto a shaft or spindle in order to facilitate positioning and affixation of abrasive saw blades relative to the shaft or spindle. Notably, Kroll does not disclose correcting for the clearance between the shaft and hub to reduce rotational imbalance in the assembly. Further, Kroll does not disclose positioning the center of mass of the hub so that it is aligned with the rotational axis of the assembly by way of a biasing element.
In order to dampen vibrations that may occur in rotating machinery as a result of the clearance between one or more rotatable elements and a shaft or spindle upon which it is installed, biasing elements, such as screws, bolts, pins or the like, may be used to more rigidly secure the rotatable element to the shaft or spindle. For instance, U.S. Pat. No. 5,463,861 to Lorenz (hereinafter “Lorenz”) discloses an apparatus relating to a friction false twist unit for crimping synthetic fibers. Damping elements are positioned about shaft bearings to dampen the vibrations generated by the rotating shaft during acceleration. In addition, a friction dampener is provided that allows radial play during high vibration and rigidly fixes the shaft during low vibration. However, Lorenz does not teach that the damping elements or the friction dampener are useful for positioning a center of mass of a rotatable element relative to an axis of rotation of a shaft or spindle onto which the rotatable element is installed.
Thus, it can be understood that reducing rotational imbalance by aligning the center of mass of high-speed rotating members to the rotational axis of the assembly is of great importance in balancing rotating assemblies and thus to the longevity and performance of rotating machinery. Nonetheless, the current state of the art lacks an element that reduces clearance between a rotatable element and a shaft or spindle upon which the rotatable element is to be positioned.